The materials accessible through the Internet are materials considered useful for understanding Japan's past relation with various neighboring Asian and other countries and regions. These materials are possessed by several Japanese Governments’ institutions and are available to the public spanning from the beginning of the Meiji era to the end of the World War II. JACAR currently provides access through the Internet to official documents of 3 governmental institutions. These official documents are sorted according to the cataloging systems of respective institutions, and compiled in "files" of varying formats and units of item or period, etc. The files are further sorted into individual historical record "items" with their own titles, according to their subject matter.

Among the collection of the National Archives of Japan, JACAR provides access to the materials of the Japanese Cabinet from the Meiji era to closing years of World War II. As these materials also include vast amounts of records relating to genuine domestic affairs, we are extracting those that concern to Asian history for the convenience of the user. We are providing "Kobun Zassan" (materials received by the Cabinet dating from the 19th year of the Meiji era (1886) to the 20th year of Showa era, but not compiled in “Kobun Ruishu”). "Goshomei Gempon" (original documents promulgating laws and treaties, with sign and seal of the Emperor), "Sumitsuin Kaigi Kankei Shorui" (document related to Privy Council meetings), and others.

JACAR provides the access through the Internet to the materials collected and made available to the public by The Diplomatic Record Archives, which are generally referred as "The Foreign Ministry records". These are records on diplomatic activities from the early years of the Meiji era to the end of World War II, including telegrams and official correspondences exchanged between Foreign Ministry and embassies and legations abroad, reports of investigation, materials on policy making, and other valuable diplomatic documents and materials. Unfortunately some records were destroyed by the fire of the Ministry of Foreign Affairs in 1942, air raids in 1945, and documents burning after the end of World War II. A part of records were requisitioned by the U.S. armed forces and seized as evidences for the Military Tribunal for the Far East. Records transcribed by Matsumoto Tadao who served as Parliamentary Vice Minister from 1937 to 1939 fortunately include a part of materials lost and these are accessible through the Internet as the "Matsumoto Record." "The Foreign Ministry records" are divided into "Meiji and Taisho eras" and "early Showa-War period" according to their system of classification.

The materials collected and made available to the public by National Institute for Defense Studies of the Ministry of Defense are composed with the documents of the former Japanese Army and Navy. The former Japanese Army and Navy destroyed large quantities of their materials at the end of the Pacific War. In the chaos of the end of the war, a lot of valuable materials were scattered and lost as well. Many official documents of the Army Ministry and Navy Ministry that escaped war flames were seized by the U.S. armed forces and placed under the control of State Department in Washington. At present, this institute is holding these former Japanese Army and Navy materials seized by the United States at the end of the war and returned in 1958 (materials returned by U.S. Government), materials maintained and stored by the "Demobilization Bureau" of the Ministry of Public Welfare, and those collected by The National Institute for Defense Studies itself, amounting to approx. 57,000 files relating to the former Japanese Army and 36,000 files relating to the former Japanese Navy.